This invention relates to a control arrangement for controlling the chilled liquid temperature in a refrigeration system, of the type having a liquid chiller, in order to minimize energy consumption while avoiding inefficient and detrimental on-off cycling of the system.
Large commercial and industrial air conditioning systems typically employ centrifugal liquid chillers. As the refrigerant flows through the system's evaporator, circulating liquid (usually water), which is in heat exchange relationship with the refrigerant, transfers heat to the refrigerant. The chilled liquid leaving the evaporator is then delivered to remote locations and used to cool a building or a zone. By maintaining the temperature of the leaving chilled liquid at a desired setpoint, the cooled space may be held at a desired temperature. The required control is usually accomplished by adjusting the position of the guide vanes or prerotation vanes, at the inlet of the system's centrifugal compressor, in response to the leaving chilled liquid temperature which is sensed. Adjusting the prerotation vanes varies the capacity of the centrifugal compressor, which in turn changes the refrigeration capacity of the system. Normally, the prerotation vanes will be adjusted, under the control of the sensed temperature, so that the leaving chilled liquid will remain close to or slightly below the desired setpoint. If the temperature of the chilled liquid drops below the setpoint by a fixed differential (such as by 4.degree. F.), the cooling load will be satisfied and the unit will be cycled off; specifically the motor driving the compressor will be de-energized thereby shutting the compressor down. (All temperatures mentioned herein will be F. or Fahrenheit.) This low temperature (4.degree. below the setpoint) is called the cut-out temperature or the low water temperature cut-out (LWT) when water is employed in the chiller. Cycling off at the cut-out temperature not only conserves energy but is a safety precaution to prevent freeze-ups.
To provide additional energy savings, many users of centrifugal chillers employ devices such as building energy management systems to reset the chilled liquid setpoint upward during periods of light load and/or heavy electrical demand. If the new reset setpoint is higher than the old desired setpoint by an amount greater than the 4.degree. fixed differential, the unit will cycle off since the actual liquid (water) temperature will be less than the new LWT setting or the reset cut-out temperature. For example, assume that the desired setpoint is 50.degree.. Under normal conditions the water will be cooled down to 46.degree. before the chiller is cycled off. As the water warms up the unit will restart at the setpoint (50.degree.) so that the water temperature will be maintained between 46.degree. and 50.degree.. Assume now that the building energy management system resets the setpoint to 58.degree., the new cut-out or LWT therby being 54.degree.. Since the water temperature will be between 46.degree. and 50.degree., and thus below the new LWT, when the system is reset, the centrifugal compressor will be shut down immediately upon reset. This will be a nuisance trip or shutdown.
Frequent stopping and starting of the centrifugal compressor and its driving motor is not recommended and is not energy efficient. This stresses the motor and shortens its life. Stopping and restarting the centrifugal liquid chiller unit by reset of the leaving water temperature setpoint is thus detrimental.
This shortcoming of the prior reset systems for the leaving chilled liquid has now been overcome by the present invention. Resetting is achieved without causing undesirable nuisance shutdowns, thereby maximizing energy savings and efficiency and extending the motor life.